Proper regulation of the PI-3 kinase (PI3k) pathway is critical for normal T cell development, activation and homeostasis. Several negative regulators of this pathway have been extensively characterized, two of which are tumor suppressors. Recently, another negative regulator of the PI3k pathway has been identified. PIK3IP1 is a transmembrane protein that has the ability to bind the catalytic protein p110 and prevent its activation. Thus far, nothing is known about the possible role of PIK3IP1 in the regulation of lymphocyte development or activation. We have found that PIK3IP1 is expressed in T cells and that ectopic expression of PIK3IP1 inhibits TCR/CD28 signaling to inducible transcription. Conversely, siRNA- mediated silencing of pik3ip1 augments activation of the same pathways. These results suggest that the PI3k regulator PIK3IP1 plays an important role in T cell activation. We will therefore determine the effects of PIK3IP1 on specific signaling pathways downstream of the TCR and CD28, including the role of specific domains within PIK3IP1 for this activity. We will also examine the expression of PIK3IP1 message and protein throughout T cell development in normal mice. Finally, we will characterize an inducible knockout of PIK3IP1 lacking expression of the protein in the T cell lineage.